Kernel driver dell-smm-hwmon¶

Copyright:: © 2002-2005 Massimo Dal Zotto <dz@debian.org>
Copyright:: © 2019 Giovanni Mascellani <gio@debian.org>

Description¶

On many Dell laptops the System Management Mode (SMM) BIOS can be queried for the status of fans and temperature sensors. Userspace utilities like sensors can be used to return the readings. The userspace suite i8kutils can also be used to read the sensors and automatically adjust fan speed (please notice that it currently uses the deprecated /proc/i8k interface).

`sysfs` interface¶

Temperature sensors and fans can be queried and set via the standard hwmon interface on sysfs, under the directory /sys/class/hwmon/hwmonX for some value of X (search for the X such that /sys/class/hwmon/hwmonX/name has content dell_smm). A number of other attributes can be read or written:

Name	Perm	Description
fan[1-3]_input	RO	Fan speed in RPM.
fan[1-3]_label	RO	Fan label.
fan[1-3]_min	RO	Minimal Fan speed in RPM
fan[1-3]_max	RO	Maximal Fan speed in RPM
fan[1-3]_target	RO	Expected Fan speed in RPM
pwm[1-3]	RW	Control the fan PWM duty-cycle.
pwm1_enable	WO	Enable or disable automatic BIOS fan control (not supported on all laptops, see below for details).
temp[1-10]_input	RO	Temperature reading in milli-degrees Celsius.
temp[1-10]_label	RO	Temperature sensor label.

Due to the nature of the SMM interface, each pwmX attribute controls fan number X.

Disabling automatic BIOS fan control¶

On some laptops the BIOS automatically sets fan speed every few seconds. Therefore the fan speed set by mean of this driver is quickly overwritten.

There is experimental support for disabling automatic BIOS fan control, at least on laptops where the corresponding SMM command is known, by writing the value 1 in the attribute pwm1_enable (writing 2 enables automatic BIOS control again). Even if you have more than one fan, all of them are set to either enabled or disabled automatic fan control at the same time and, notwithstanding the name, pwm1_enable sets automatic control for all fans.

If pwm1_enable is not available, then it means that SMM codes for enabling and disabling automatic BIOS fan control are not whitelisted for your hardware. It is possible that codes that work for other laptops actually work for yours as well, or that you have to discover new codes.

Check the list i8k_whitelist_fan_control in file drivers/hwmon/dell-smm-hwmon.c in the kernel tree: as a first attempt you can try to add your machine and use an already-known code pair. If, after recompiling the kernel, you see that pwm1_enable is present and works (i.e., you can manually control the fan speed), then please submit your finding as a kernel patch, so that other users can benefit from it. Please see Documentation/process/submitting-patches.rst for information on submitting patches.

If no known code works on your machine, you need to resort to do some probing, because unfortunately Dell does not publish datasheets for its SMM. You can experiment with the code in this repository to probe the BIOS on your machine and discover the appropriate codes.

Again, when you find new codes, we’d be happy to have your patches!

`thermal` interface¶

The driver also exports the fans as thermal cooling devices with type set to dell-smm-fan[1-3]. This allows for easy fan control using one of the thermal governors.

Module parameters¶

force:bool
Force loading without checking for supported models. (default: 0)
ignore_dmi:bool
Continue probing hardware even if DMI data does not match. (default: 0)
restricted:bool
Allow fan control only to processes with the CAP_SYS_ADMIN capability set or processes run as root when using the legacy /proc/i8k interface. In this case normal users will be able to read temperature and fan status but not to control the fan. If your notebook is shared with other users and you don’t trust them you may want to use this option. (default: 1, only available with CONFIG_I8K)
power_status:bool
Report AC status in /proc/i8k. (default: 0, only available with CONFIG_I8K)
fan_mult:uint
Factor to multiply fan speed with. (default: autodetect)
fan_max:uint
Maximum configurable fan speed. (default: autodetect)

Legacy `/proc` interface¶

Warning

This interface is obsolete and deprecated and should not used in new applications. This interface is only available when kernel is compiled with option CONFIG_I8K.

The information provided by the kernel driver can be accessed by simply reading the /proc/i8k file. For example:

$ cat /proc/i8k
1.0 A17 2J59L02 52 2 1 8040 6420 1 2

The fields read from /proc/i8k are:

1.0 A17 2J59L02 52 2 1 8040 6420 1 2
|   |   |       |  | | |    |    | |
|   |   |       |  | | |    |    | +------- 10. buttons status
|   |   |       |  | | |    |    +--------- 9.  AC status
|   |   |       |  | | |    +-------------- 8.  fan0 RPM
|   |   |       |  | | +------------------- 7.  fan1 RPM
|   |   |       |  | +--------------------- 6.  fan0 status
|   |   |       |  +----------------------- 5.  fan1 status
|   |   |       +-------------------------- 4.  temp0 reading (Celsius)
|   |   +---------------------------------- 3.  Dell service tag (later known as 'serial number')
|   +-------------------------------------- 2.  BIOS version
+------------------------------------------ 1.  /proc/i8k format version

A negative value, for example -22, indicates that the BIOS doesn’t return the corresponding information. This is normal on some models/BIOSes.

For performance reasons the /proc/i8k doesn’t report by default the AC status since this SMM call takes a long time to execute and is not really needed. If you want to see the ac status in /proc/i8k you must explictitly enable this option by passing the power_status=1 parameter to insmod. If AC status is not available -1 is printed instead.

The driver provides also an ioctl interface which can be used to obtain the same information and to control the fan status. The ioctl interface can be accessed from C programs or from shell using the i8kctl utility. See the source file of i8kutils for more information on how to use the ioctl interface.

SMM Interface¶

Warning

The SMM interface was reverse-engineered by trial-and-error since Dell did not provide any Documentation, please keep that in mind.

The driver uses the SMM interface to send commands to the system BIOS. This interface is normally used by Dell’s 32-bit diagnostic program or on newer notebook models by the buildin BIOS diagnostics. The SMM may cause short hangs when the BIOS code is taking too long to execute.

The SMM handler inside the system BIOS looks at the contents of the eax, ebx, ecx, edx, esi and edi registers. Each register has a special purpose:

Register	Purpose
eax	Holds the command code before SMM, holds the first result after SMM.
ebx	Holds the arguments.
ecx	Unknown, set to 0.
edx	Holds the second result after SMM.
esi	Unknown, set to 0.
edi	Unknown, set to 0.

The SMM handler can signal a failure by either:

setting the lower sixteen bits of eax to 0xffff
not modifying eax at all
setting the carry flag (legacy SMM interface only)

Legacy SMM Interface¶

When using the legacy SMM interface, a SMM is triggered by writing the least significant byte of the command code to the special ioports 0xb2 and 0x84. This interface is not described inside the ACPI tables and can thus only be detected by issuing a test SMM call.

WMI SMM Interface¶

On modern Dell machines, the SMM calls are done over ACPI WMI:

#pragma namespace("\\\\.\\root\\dcim\\sysman\\diagnostics")
[WMI, Provider("Provider_DiagnosticsServices"), Dynamic, Locale("MS\\0x409"),
 Description("RunDellDiag"), guid("{F1DDEE52-063C-4784-A11E-8A06684B9B01}")]
class LegacyDiags {
 [key, read] string InstanceName;
 [read] boolean Active;

 [WmiMethodId(1), Implemented, read, write, Description("Legacy Method ")]
 void Execute([in, out] uint32 EaxLen, [in, out, WmiSizeIs("EaxLen") : ToInstance] uint8 EaxVal[],
              [in, out] uint32 EbxLen, [in, out, WmiSizeIs("EbxLen") : ToInstance] uint8 EbxVal[],
              [in, out] uint32 EcxLen, [in, out, WmiSizeIs("EcxLen") : ToInstance] uint8 EcxVal[],
              [in, out] uint32 EdxLen, [in, out, WmiSizeIs("EdxLen") : ToInstance] uint8 EdxVal[]);
};

Some machines support only the WMI SMM interface, while some machines support both interfaces. The driver automatically detects which interfaces are present and will use the WMI SMM interface if the legacy SMM interface is not present. The WMI SMM interface is usually slower than the legacy SMM interface since ACPI methods need to be called in order to trigger a SMM.

SMM command codes¶

Command Code	Command Name	Description
`0x0025`	Get Fn key status	Returns the Fn key pressed after SMM: 9th bit in `eax` indicates Volume up 10th bit in `eax` indicates Volume down both bits indicate Volume mute
`0xa069`	Get power status	Returns current power status after SMM: 1st bit in `eax` indicates Battery connected 3th bit in `eax` indicates AC connected
`0x00a3`	Get fan state	Returns current fan state after SMM: 1st byte in `eax` holds the current fan state (0 - 2 or 3)
`0x01a3`	Set fan state	Sets the fan speed: 1st byte in `ebx` holds the fan number 2nd byte in `ebx` holds the desired fan state (0 - 2 or 3)
`0x02a3`	Get fan speed	Returns the current fan speed in RPM: 1st byte in `ebx` holds the fan number 1st word in `eax` holds the current fan speed in RPM (after SMM)
`0x03a3`	Get fan type	Returns the fan type: 1st byte in `ebx` holds the fan number 1st byte in `eax` holds the fan type (after SMM): 5th bit indicates docking fan 1 indicates Processor fan 2 indicates Motherboard fan 3 indicates Video fan 4 indicates Power supply fan 5 indicates Chipset fan 6 indicates other fan type
`0x04a3`	Get nominal fan speed	Returns the nominal RPM in each fan state: 1st byte in `ebx` holds the fan number 2nd byte in `ebx` holds the fan state in question (0 - 2 or 3) 1st word in `eax` holds the nominal fan speed in RPM (after SMM)
`0x05a3`	Get fan speed tolerance	Returns the speed tolerance for each fan state: 1st byte in `ebx` holds the fan number 2nd byte in `ebx` holds the fan state in question (0 - 2 or 3) 1st byte in `eax` returns the speed tolerance
`0x10a3`	Get sensor temperature	Returns the measured temperature: 1st byte in `ebx` holds the sensor number 1st byte in `eax` holds the measured temperature (after SMM)
`0x11a3`	Get sensor type	Returns the sensor type: 1st byte in `ebx` holds the sensor number 1st byte in `eax` holds the temperature type (after SMM): 1 indicates CPU sensor 2 indicates GPU sensor 3 indicates SODIMM sensor 4 indicates other sensor type 5 indicates Ambient sensor 6 indicates other sensor type
`0xfea3`	Get SMM signature	Returns Dell signature if interface is supported (after SMM): `eax` holds 1145651527 (0x44494147 or “DIAG”) `edx` holds 1145392204 (0x44454c4c or “DELL”)
`0xffa3`	Get SMM signature	Same as `0xfea3`, check both.

There are additional commands for enabling (0x31a3 or 0x35a3) and disabling (0x30a3 or 0x34a3) automatic fan speed control. The commands are however causing severe sideeffects on many machines, so they are not used by default.

On several machines (Inspiron 3505, Precision 490, Vostro 1720, ...), the fans supports a 4th “magic” state, which signals the BIOS that automatic fan control should be enabled for a specific fan. However there are also some machines who do support a 4th regular fan state too, but in case of the “magic” state, the nominal RPM reported for this state is a placeholder value, which however is not always detectable.

Firmware Bugs¶

The SMM calls can behave erratic on some machines:

Firmware Bug

Affected Machines

Reading of fan states return spurious errors.

Precision 490

Reading of fan types causes erratic fan behaviour.

Studio XPS 8000

Studio XPS 8100

Inspiron 580

Inspiron 3505

Fan-related SMM calls take too long (about 500ms).

Inspiron 7720

Vostro 3360

XPS 13 9333

XPS 15 L502X

In case you experience similar issues on your Dell machine, please submit a bugreport on bugzilla to we can apply workarounds.

Limitations¶

The SMM calls can take too long to execute on some machines, causing short hangs and/or audio glitches. Also the fan state needs to be restored after suspend, as well as the automatic mode settings. When reading a temperature sensor, values above 127 degrees indicate a BIOS read error or a deactivated sensor.

The Linux Kernel

Contents

This Page

Kernel driver dell-smm-hwmon¶

Description¶

`sysfs` interface¶

Disabling automatic BIOS fan control¶

`thermal` interface¶

Module parameters¶

Legacy `/proc` interface¶

SMM Interface¶

Legacy SMM Interface¶

WMI SMM Interface¶

SMM command codes¶

Firmware Bugs¶

Limitations¶

Kernel driver dell-smm-hwmon¶

Description¶

sysfs interface¶

Disabling automatic BIOS fan control¶

thermal interface¶

Module parameters¶

Legacy /proc interface¶

SMM Interface¶

Legacy SMM Interface¶

WMI SMM Interface¶

SMM command codes¶

Firmware Bugs¶

Limitations¶

`sysfs` interface¶

`thermal` interface¶

Legacy `/proc` interface¶